:condition-case key should be able to specify the error-symbol

[vermiculus]

Right now, the :condition-case key doesn't provide any symbol for error handlers. This will soon change to accommodate changes in Ghub, but due to time constraints, I can't change it in the way I'd like.

Ideally, we want syntax like this:

:condition-case err
((sym-1 (handle-1 err))
 (sym-2 (handle-2 err))

This is difficult because at the moment, these keys are expected to have one value. Perhaps a different key should be introduced to specify the error-symbol, but I'd like to think on that more.

[vermiculus]

Related to conversations in magit/ghub@6fd4434